52 CARNEGIE INSTITUTION OF WASHINGTON. 



centration, of which carefully measured volumes were drawn through the 

 BA(0H)2 solution and the weight of CO2 absorbed was calculated. 



Thus, e. g., duplicate determinations of CO2 in 3 and 5 liters of air showed 

 a difference of 0.05 and 0.11 mg. CO2, respectively. These differences are 

 equivalent to 0.001 per cent by volume of CO2 in the determinations, an order 

 of accuracy obtained only with the Sond^n apparatus. From the data so 

 obtained, knowing the original concentration of the solution, the CO2 gram 

 equivalent of each 75-c. c. portion was calculated after successive definite 

 volumes of air had been drawn through. Thus a concentration-resistance 

 curve was constructed in which specific resistance was plotted against CO2 

 gram equivalents of 75 c. c. Ba(0H)2. The amount of CO2 absorbed by 

 75 c. c. Ba(0H)2 could thus be read directly from the curve. 



The leaves used in the work on photosynthesis rarely have a rate of CO2- 

 emission of less than 0.5 mg. CO2 per hour at 25°. Using a cell with a con- 

 stant of 85.204, a difference of 1 ohm in the observed resistance of the 

 Ba(0H)2 solution, when calculated as specific resistance, is equal to 0.45 mg. 

 CO2 on the middle portion of the curve. Hence a milligram of CO2, when 

 absorbed, would change the observed resistance of the solution 21.7 ohms. 

 The differences in the rates of CO2 emission or fixation by a single leaf in a 

 half-hour period thus fall well within the accuracy of the method. 



Effect of Fluctuations in the CO^-content of the Atmosphere on the Rate of 

 Respiration of Leaves, by H. A. Spoehr and J. M. McGee. 



For practical experimental purposes it is often desirable that the photo- 

 synthesis determinations be made in an atmosphere enriched in CO2. In 

 determining the rate of photosynthesis on the basis of CO2 fixed, it is necessary 

 also to determine the rate of respiration. It has been very generally assumed 

 that the latter, determined in the dark, remains the same during illumination. 

 A correct estimation of the rate of photosynthesis depends, therefore, largely 

 upon an exact determination of the respiratory value. Moreover, the rate of 

 respiration is very often determined by measuring the CO2 emitted in a 

 stream of air free of CO2. It has been found, however, that changes in the 

 partial pressure of the CO2 surrounding the plant have a profound influence 

 on the rate of C02-emission. 



When the C02-content of the air surrounding a leaf is changed from a lower 

 to a higher concentration, the leaf shows a reduced rate of C02-emission for a 

 period following the change, then increases, and finally again attains about the 

 same rate as before the change in C02-content was made. Conversely, when 

 the C02-content of the air surrounding a leaf is changed from a higher to a 

 lower concentration, the leaf shows a primary increased rate of C02-emission 

 and subsequent decrease to the original rate. 



The intensity of this increased or decreased rate varies with different 

 species of leaves (Helianthus annuus, Echinocystis fabacea, Hydrangea hortensis) 

 as does also the duration of the effect of the change. These results substantiate 

 the opinion that determinations of the rate of photosynthesis in a closed 

 system of air yield spurious results. 



The same is true when using aquatic plants where a slightly alkaline solu- 

 tion is produced during photosynthesis. The effect can be avoided either by 

 using a very rapid air-stream, which introduces the difficulty of complete CO2- 

 absorption, or in the case of aquatic plants by employing a carefully adjusted 

 buffer solution. While the results can in part be explained on the basis of the 



